1. Field of the Invention
The present invention relates to magnetically driven positioning devices. A more particular aspect of the invention concerns an inexpensive system and method for determining the orientation of a coil driven actuator arm in a magnetic disk drive without the aid of servo signals from the disk.
2. Description of the Related Art
During normal operation of a data storage disk drive, such as a magnetic disk drive, a read/write head senses servo signals stored on a disk as the read head glides across the disk surface. A servo controller interprets the servo signals, and uses these signals to adjust the head's position relative to the disk surface. The servo controller moves the head, either to maintain a desired head position or to travel to a new position, by moving an actuator arm whose tip is secured to the head.
During certain situations, however, servo signals are not available to guide or position the head. For instance, the operation of "parking" the head moves the head away from the region of the disk surface containing data. Consequently, guidance of the head to and from a "park" position cannot be conducted using servo signals.
To overcome this problem, various known methods estimate head velocity by analyzing certain electrical characteristics of an actuator's voice coil motor ("VCM"), which is used to position the actuator arm. Broadly, the VCM includes a wound conductive coil (called a "voice coil") secured to the actuator arm, and one or more permanent magnets. The coil is positioned within the magnetic field of the permanent magnets. Thus, applying a current through the voice coil creates a magnetic force that moves the actuator arm relative to the permanent magnet.
Some known methods estimate the velocity of the actuator arm using the back-emf voltage ("V.sub.bemf " or "back-emf") present across the voice coil. The back-emf is the voltage drop across the voice coil due to the coil's motion through the field of the permanent magnets. The back-emf is therefore the inductive component of the voltage drop across the voice coil. The back-emf is proportional to the actuator arm's velocity in the ratio of a known constant.
One approach to estimating the arm's velocity involves the following steps. First, the total voice coil voltage is measured, then this sum is converted to the digital domain with an analog-to-digital ("A/D") converter. Next, the calculated ohmic portion of the coil voltage is digitally subtracted, leaving the inductive portion of the coil voltage, i.e., the back-emf. The ohmic portion of the coil voltage may be calculated using a previous measurement of coil resistance and a measurement of the coil current. The resistance is usually measured while the actuator arm is stationary, by measuring the voltage across the voice coil while applying a VCM current that produces no actuator motion.
Although this approach may be adequate for some applications, it may be unsatisfactory for disk drives that cannot afford the expense, space, or processing required to operate an A/D converter.
Another approach estimates back-emf by measuring total coil voltage, and subtracting in the analog domain an estimate of the portion of the coil voltage attributable to coil resistance. This estimate is generated by sampling the voltage across a sense resistor coupled in series with the voice coil, and multiplying this voltage by the ratio between a nominal coil resistance and the nominal sense resistance. This approach is explained in Eaton et al., "Improved Back Electromagnetic Force Voice Coil Motor Controller", IBM Technical Disclosure Bulletin, Vol. 38, No. 11, November 1995. This reference is incorporated by reference in its entirety.
Although useful in some circumstances, the Eaton approach does not account for variations in coil resistance from the estimated coil resistance. Such variations may result from a number of different conditions. For example, normal disk drive operations typically involve heating and cooling of the voice coil, which cause temperature-dependent excursions in the voice coil resistance. In addition, voice coil resistance may also vary from anticipated values due to manufacturing variations. As a result, estimations of back-emf dependent upon a fixed value of coil resistance are not accurate under all circumstances.
This potential inaccuracy is further compounded while removing an actuator arm from a parked position, since the current through the voice coil, and hence the voltage drop due to resistance of the coil, is substantially greater than the current during other actuator movement. In particular, breaking free from a magnetic latch can require large forces: with a coil current on the order of one amp, and a deviation in coil resistance of .+-.two ohms, the estimated back-emf may be two volts off. In some systems, this may lead to a velocity estimate having an error of .+-.one meter per second.